The invention relates to a cathode ray tube provided with at least one oxide cathode comprising a cathode carrier with a cathode base of a cathode metal and a cathode coating of an electron-emitting material containing barium, a further alkaline earth oxide and yttrium oxide, scandium oxide or a rare earth metal oxide in the form of oxide particles.
A cathode ray tube is composed of 4 functional groups:
electron beam generation in the electron gun,
beam focusing using electrical or magnetic lenses,
beam deflection to generate a raster, and
luminescent screen or display screen.
The functional group relating to electron beam generation comprises an electron-emitting cathode, which generates the electron current in the cathode ray tube and which is enclosed by a control grid, for example a Wehnelt cylinder having an apertured diaphragm on the front side.
An electron-emitting cathode for a cathode ray tube generally is a punctiform, heatable oxide cathode with an electron-emitting, oxide-containing cathode coating. If an oxide cathode is heated, then electrons are evaporated from the electron-emitting coating into the surrounding vacuum. If the Wehnelt cylinder is biased with respect to the cathode, then the quantity of emergent electrons and hence the beam current of the cathode ray tube can be controlled.
The quantity of electrons that can be emitted by the cathode coating depends on the work function of the electron-emitting material. Nickel, which is customarily used for the cathode base, has itself a comparatively high work function. For this reason, the metal of the cathode base is customarily coated with another material, which mainly serves to improve the electron-emitting properties of the cathode base. A characteristic feature of the electronemitting coating materials of oxide cathodes is that they comprise an alkaline earth metal in the form of the alkaline earth metal oxide.
To manufacture an oxide cathode, a suitably shaped nickel sheet is coated, for example, with the carbonates of the alkaline earth metals in a binder preparation. During evacuating and baking out the cathode ray tube, the carbonates are converted to the alkaline earth metal oxides at temperatures of approximately 1000xc2x0 C. After this bum-off of the cathode, said cathode already supplies a noticeable emission current which, however, is still unstable. Next, an activation process is carried out. This activation process causes the originally non-conducting ionic lattice of the alkaline earth oxides to be converted to an electronic semiconductor in that donor-type impurities are incorporated in the crystal lattice of the oxides. These impurities essentially consist of elementary alkaline earth metal, for example calcium, strontium or barium. The electron emission of such oxide cathodes is based on the impurity mechanism. Said activation process serves to provide a sufficiently large quantity of excess, elementary alkaline earth metal, which enables the oxides in the electron-emitting coating to supply the maximum emission current at a prescribed heating capacity. A substantial contribution to the activation process is made by the reduction of barium oxide to elementary barium by alloy constituents (xe2x80x9cactivatorsxe2x80x9d) of the nickel from the cathode base.
For the function and the service life of an oxide cathode it is important that elementary alkaline earth metal is continuously dispensed. The reason for this being that the cathode coating continuously loses alkaline earth metal during the service life of the cathode. The cathode material partly evaporates slowly and is partly sputtered off by the ion current in the lamp.
However, initially the elementary alkaline earth metal is continuously dispensed. Said dispensation stops, however, when a thin, yet high-impedance interface of alkaline earth silicate or alkaline earth aluminate forms between the cathode base and the emitting oxide in the course of time.
The service life is also influenced by the fact that the amount of activator metal in the nickel alloy of the cathode base becomes depleted in the course of time.
U.S. Pat. No. 5,075,589 issued 24 Dec. 1991 to Dirks et al. discloses an oxide cathode comprising a carrier body, which is essentially composed of nickel, and a layer of an electron-emitting material containing alkaline earth oxide including barium and maximally 5% by weight yttrium oxide, scandium oxide or rare earth metal oxide, said yttrium oxide, scandium oxide and the rare earth metal oxide being particles, the majority of which have a diameter of maximally 5 xcexcm.
It is an object of the invention to provide a cathode ray tube, the beam current of which is uniform and remains constant for a long period of time, while said cathode ray tube can be reproducibly manufactured.
In accordance with the invention, this object is achieved by a cathode ray tube provided with at least one oxide cathode comprising a cathode carrier with a cathode base of a cathode metal and a cathode coating of an electron-emitting material containing a particle-particle composite material of oxide particles of an alkaline earth oxide selected from the group formed by the oxides of calcium, strontium and barium, and oxide particles having a first grain size distribution of an oxide selected from the group formed by the oxides of scandium, yttrium and the lanthanoids, and oxide particles having a second grain size distribution of an oxide selected from the group formed by the oxides of scandium, yttrium and the lanthanoids.
Cathode ray tubes comprising such an oxide cathode have a uniform beam current for a long period of time, which can be attributed to the fact that the bimodal grain size distribution of the oxide particles of the oxides of scandium, yttrium and the lanthanoids in the electron-emitting material of the cathode coating causes the initial emission to be high already while the resistance to oxygen poisoning is low.
The bimodal grain size distribution also leads to an increase of the Ba retention. The cathode is not susceptible to ion bombardment, its emission is uniform and it can be reproducibly manufactured.
As barium is dispensed continuously, depletion of the electron emission, as known from the oxide cathodes according to the prior art, is precluded. Substantially higher beam current densities can be obtained without adversely affecting the service life. This can also be used to draw the necessary electron beam currents from smaller cathode regions. The spot size of the hot spot determines the beam focusing quality on the display screen. The picture definition is increased throughout the screen. As, in addition, the cathodes are not subject to aging, picture brightness and picture definition can be maintained at a high level throughout the service life of the tube.
Within the scope of the invention it is preferred that the oxide particles having a first grain size distribution have an average grain size 0.4 less than d50 less than 5 xcexcm, and the oxide particles having a second grain size distribution have an average grain size d50xe2x89xa60.4 xcexcm.
It may alternatively be preferred that the electron-emitting material comprises the oxide particles having a first grain size distribution in a concentration in the range from 0.1 to 20 wt. %, and the oxide particles having a second grain size distribution in a concentration in the range from 1*10xe2x88x926 to 1*10xe2x88x923 wt. %.
It may be additionally preferred that the oxide particles of an alkaline earth oxide selected from the group formed by the oxides of calcium, strontium and barium are doped with an element selected from the group formed by scandium, yttrium and the lanthanoids in a quantity ranging from 0.10*10xe2x88x926 to 10*10xe2x88x926 wt. %.
In another preferred embodiment of the invention, the electron-emitting material is a stratified composite of at least a first and at least a second layer, said first layer comprising oxide particles of an alkaline earth oxide selected from the group formed by the oxides of calcium, strontium and barium, and oxide particles having a first grain size distribution of an oxide selected from the group formed by the oxides of scandium, yttrium and the lanthanoids, and said second layer comprising oxide particles of an alkaline earth oxide selected from the group formed by the oxides of calcium, strontium and barium, and oxide particles having a second grain size distribution of an oxide selected from the group formed by the oxides of scandium, yttrium and the lanthanoids.
In a further preferred embodiment of the invention, the electron-emitting material is a stratified composite of at least a first and at least a second layer, said first layer comprising oxide particles of an alkaline earth oxide selected from the group formed by the oxides of calcium, strontium and barium, and oxide particles of an oxide selected from the group formed by the oxides of scandium, yttrium and the lanthanoids having a first or a second grain size distribution in a quantity ranging from 2 to 20 wt. %, and said second layer comprising oxide particles of an alkaline earth oxide selected from the group formed by the oxides of calcium, strontium and barium, and oxide particles of an oxide selected from the group formed by the oxides of scandium, yttrium and the lanthanoids having a first or a second grain size distribution in a quantity ranging from 0.1 to 5 wt. %.
If the electron-emitting material comprises 1 to 3 wt. % particles of an activator metal selected from the group formed by Mg, Al, Fe, Si, Ti, Hf, Zr, W, Mo, Mn and Cr, or the electron-emitting material comprises 1 to 3 wt. % particles of an activator metal selected from the group formed by Mg, Al, Fe, Si, Ti, Hf, Zr, W, Mo, Mn and Cr, which are coated with a metal selected from the group formed by Pd, Rh, Pt, Co, Ni, Ir, Re, the oxide cathode combines robust behavior with rapid switching.
The invention also relates to an oxide cathode comprising a cathode carrier with a cathode base of a cathode metal and a cathode coating of an electron-emitting material comprising a particle-particle composite material of oxide particles of an alkaline earth oxide selected from the group formed by the oxides of calcium, strontium and barium, and oxide particles having a first grain size distribution of an oxide selected from the group formed by the oxides of scandium, yttrium and the lanthanoids, and oxide particles having a second grain size distribution of an oxide selected from the group formed by the oxides of scandium, yttrium and the lanthanoids.
These and other aspects of the invention will be apparent from and elucidated with reference to two embodiments described hereinafter.